One of the ways we work to reduce the squirrel population around our pecan grove is to set traps. We use #110 conibear traps (photo at right) mounted on small shelves constructed from 2 by 4 lumber and attached to tree trunks.
When setting the trap, hold the trap open and place over the nails. Squeeze the trap tightly against the nails then set the trap catch. Note that we use pecans to bait the trap but several types of nuts will work. Make sure to bait the trap before setting it ( I don't want to hear about broken fingers!).